1. Field of the Invention
The present invention relates to a method for transferring a porous layer, to a method for making a semiconductor device, and to a method for making a solar battery. In particular, the present invention relates to a method for forming a porous layer on an inexpensive substrate and to a method for making semiconductor devices, such as a solar battery, by depositing crystalline thin-films on an inexpensive substrate with a porous layer provided therebetween.
2. Description of the Related Art
Solar batteries have been widely studied as driving energy sources for various devices and as electric power sources which are connected to commercial power systems.
Low-cost production of solar batteries requires forming semiconductor devices on an inexpensive substrate. A typical semiconductor material for solar batteries is silicon. In particular, single-crystal silicon is most advantageous in view of photoelectric conversion efficiency which is an efficiency for converting optical energy into electromotive force. On the other hand, amorphous silicon is advantageous in view of production using a large substrate at low cost. In recent years, the use of polycrystalline silicon has been examined in order to achieve a low cost comparable to that of amorphous silicon and a high energy conversion efficiency comparable to that of single-crystal silicon.
Since conventional methods for making solar batteries using single-crystal silicon or polycrystalline silicon use platelet substrates which are formed by slicing a crystal ingot, it is difficult to decrease the thickness to 0.3 mm or less. Since such substrates are larger than a thickness (approximately 20 xcexcm to 50 xcexcm) which is required for sufficiently absorbing light, the substrate material is not effectively used. Accordingly, low-cost production of solar batteries requires development of thinner single-crystal silicon or polycrystalline silicon substrates.
In recent years, a method for making a silicon sheet by a spinning process has been proposed in which melted silicon droplets are cast into a mold. The thickness of the crystal silicon substrate prepared by this method reaches approximately 0.1 mm to 0.2 mm at minimum, and is still larger than the above thickness (approximately 20 xcexcm to 50 xcexcm) which is required for sufficiently absorbing light.
An attempt to accomplish both a high energy conversion efficiency and a low production cost is proposed by using an epitaxial thin-layer which is deposited on a single-crystal silicon substrate and is cleaved from the substrate as solar batteries (Milnes, A. G. and Feucht, D. L., xe2x80x9cPeeled Film Technology Solar Cellsxe2x80x9d, IEEE Photovoltaic Specialist Conference, p. 338, 1975).
In this method, however, a SiGe interlayer is provided on a single-crystal silicon substrate, a single-crystal silicon layer is heteroepitaxially deposited on the interlayer, and the interlayer is selectively melted to cleave the heteroepitaxially deposited single-crystal silicon layer. In general, defects readily occur at the interface between the underlayer and the heteroepitaxially deposited layer, since these layers have different lattice constants. Moreover, this method is not always advantageous in view of material cost since the method uses different materials.
U.S. Pat. No. 4,816,420 discloses a thin crystalline solar battery which is produced by a method including forming a crystalline sheet by selective epitaxial growth through a mask and then lateral crystal growth on a crystal substrate and cleaving the sheet from the substrate.
In this method, the mask has slits as line seeds, and a crystal sheet, which is formed by selective epitaxy from the slits and then lateral growing, is cleaved. That is, the crystal sheet is mechanically cleaved by means of crystal cleavage. When the line seeds have a size greater than a certain size, the crystal has a large contact area with the substrate, and the crystal sheet may be damaged during the cleaving process. In the production of solar batteries having a large area, the slit length may be several millimeters to several centimeters no matter how much the slit width is decreased (in practice, the slit width may be reduced to approximately 1 xcexcm). Thus, this method is not practical.
In consideration of such problems, Japanese Unexamined Patent Application Publication No. 6-45622 discloses a method for making a crystalline thin-film solar battery having satisfactory characteristics. In this method, a porous silicon layer is formed on a silicon wafer by anodization and is then cleaved from the wafer. The cleaved porous silicon layer is bonded onto a metal substrate and an epitaxial layer is deposited thereon. This product is used as the solar battery.
In this method, however, the metal substrate is exposed in a high-temperature process. Such a process may causes inclusion of impurities in the epitaxial layer, resulting in deterioration of properties as the epitaxial layer. Moreover, a significantly thin porous layer is handled alone in this process. Thus, particular attention must be directed to transportation of the thin porous layer.
Accordingly, it is an object of the present invention to provide a method for transferring a porous layer which produces a semiconductor layer having satisfactory characteristics.
It is another object of the present invention to provide a method suitable for mass production of semiconductor devices having satisfactory characteristics.
It is still another object of the present invention to provide a method suitable for mass production of high-efficiency solar batteries.
It is a further object of the present invention to reuse a crystal substrate which was used for forming a porous layer thereon, after the substrate was separated therefrom, in order to reduce production costs.
An aspect of the present invention relates to a method for transferring a porous layer including an anodization step of forming by anodization a porous layer on one side of a crystalline silicon member, a fixing step of fixing a supporting substrate onto the surface of the porous layer, and a cleaving step of applying force to any one of the supporting substrate and the porous layer, whereby at least part of the porous layer is cleaved from the crystalline silicon member and is transferred onto the supporting substrate.
Other aspects of the present invention relate to methods for making a semiconductor device and a solar battery including the above steps for transferring the porous layer onto the supporting substrate and a step of forming a crystalline semiconductor layer onto the transferred porous layer.
The terms xe2x80x9cat least part of the porous layer is cleavedxe2x80x9d mean that the porous layer is cleaved at an interface between the porous layer and the other layer of the crystalline silicon member, that part of the porous layer is cleaved so that the porous layer remains on the crystalline silicon substrate. That is, the entire porous layer and the unanodized portion are separated from each other at the interface therebetween, or part of the porous layer and the unanodized portion are separated from each other at the interior of the porous layer.
The crystalline silicon member includes a substrate, a film, and the like.
In these methods, another porous layer is preferably formed by anodization onto the crystalline silicon after the prior porous layer is transferred. Moreover, the other porous layer is preferably transferred onto another supporting substrate which is different from the prior supporting substrate.